ISPConfig 3 Monitor Ajax Request was not successful

I have been using ISPConfig 3.0.2.1 on Ubuntu 10.04 for a month now and everything was working realy smooth.

But since a couple of days I get the error 'Ajax Request was not successful' on ISPConfig when I click the Monitor tab.
All the other items are working perfectly, also the items on the left menu on the Monitor tab. Only the item 'Show overview' (the page that is loaded by default when clicking on the Monitor tab) is giving the error.

It is quite strange, because I haven't changed anything on the system the last couple of weeks.